The invention concerns the membranous mirror telescopes.
The U.S. application Ser. No. 08/809,620, describe a telescope made up of a mirror storey, a focal storey and a storey of control of the mirror.
The mirror storey comprises a membranous mirror and an actuating membrane, Jointed by a spacer.
For their folding, the parabolic mirror (or of another shape) and the associated actuating membrane are put in contact and are made quasi plane by a succession of alternated concentric deformations, and the quasi plane so obtained is rolled up on itself one or more time to form a cylinder.
A radio telescope called Inflatable Space Rigidizing Structure (ISRS) was put into orbit in 1996.
It is made up of a reflective membrane and of a passive membrane, the two membranes being jointed by their peripheries.
For its orbiting the ISRS was folded in a disordered way.
D. Daymon, U.S. Pat. No. 3,151,206, and D. K. Marker. OPTICS EXPRESS. 1 n 11 (1997), disclose various arrangements for mirror devices.
Disadvantages of the Former Art.
The disadvantage of the folding of the mirror and the actuating membrane as described in the U.S. request Ser. No. 08/809,620 is the difficulty in keeping without artifice its quasi flatness of folding and its rolling up on itself during orbiting.
The disadvantage of the mode of disordered folding of the ISRS is that this type of folding generated permanent deformations strongly decreasing the effectiveness of the said ISRS.
Auxiliary Membrane of Folding.
To facilitate the folding of mirror 1 and its actuating membrane 2, there exists, according to the invention, a preformed auxiliary membrane 3 comprising concentric undulations onto which the actuating membrane and the mirror will apply, and that  less than we greater than  we will call auxiliary of folding; we will call optical unit the unit consisted the mirror, the actuating membrane and the auxiliary of folding.
These undulations consist of a concentric succession of circular elements of which the developed length of the generant is equal to the developed length of the generant 4 of mirror 1, and the peripheral diameter equal to the diameter of the mirror.
In a preferred realization, each undulation of the generant is with a minimal curvature.
This preformed auxiliary of folding, after unfolding, could be fastened by its periphery, with elastic damped means, to the top of the low part of the telescope.
This fixing will ensure the plane shape of the auxiliary of folding and the circularity of the top of this low element of the telescope.
Electric circuits integrated into this auxiliary of folding take part in the positioning of the actuating membrane